Clear
evidence for resonant tunnelling of electrons in a p-i-n quantum well modulator is
provided by picosecond pump-and-probe electro-absorption measurements. The
temperature-independent escape times show a drastic reduction when an electrical field is
applied perpendicular to the wells, with a pronounced minimum at the field corresponding
to the resonance between the n=1 electron level in one quantum well, and the n=2 electron
level in the adjacent one. The authors' calculated field dependence of the electron
tunnelling times proves that this behaviour is the signature of resonant tunnelling